1. Field of the Invention
The present invention generally relates to a fault detection unit which is used for a rotation angle detecting device for detecting a rotation angle of a rotor with respect to a stator and detects the occurrence of a fault in the device.
2. Description of Related Art
As well known, a rotational angle detecting device is used to detect a rotation angle of a rotor with respect to a stator by using a resolver. In this device, an exciting signal having a predetermined periodic waveform is supplied to a resolver, a sine-wave phase signal and a cosine-wave phase signal are output from the resolver in response to the exciting signal, and each of the phase signals is amplitude-modulated in a sinusoidal shape to extract a sine-wave phase amplitude signal As(θ) and a cosine-wave phase amplitude signal Ac(θ) from the phase signals, respectively. Each amplitude signal has an amplitude changed with the rotational angle, and a rotational angle θ of a rotor with respect to a stator is calculated from the amplitude signals.
Further, a unit for detecting a fault of the resolver used in the rotational angle detecting device is known. For example, Published Japanese Patent First Publication No. H09-72758 has disclosed a resolver fault detection unit. In this unit, a square of amplitude is calculated for each amplitude signal. When it is detected that a sum As(θ)2+As(θ)2 of the squared amplitudes or a square root of the sum is out of a predetermined range, it is judged that a fault has occurred in the resolver.
However, in this unit, even if a fault has actually occurred in the resolver, there is a case where the sum or the square root is within the range. Therefore, in this case, it is erroneously judged that no fault has occurred in the resolver. In contrast, even if no fault has actually occurred in the resolver, there is a case where, although the unit can detect the rotational angle θ, the unit cannot temporarily detect the amplitude signals correctly. Therefore, in this case, it is erroneously judged that a fault has occurred in the resolver.
To solve this problem, Published Japanese Patent First Publication No. 2006-177750 has disclosed another resolver fault detection unit for a rotational angle detecting device. In this Publication, the unit calculates the rotational angle θ and a square root of a sum As(θ)2+As(θ)2 squared amplitudes from the amplitude signals As(θ) and As(θ). When the unit detects that the square root is out of a predetermined range, the unit judges that the resolver is in a tentative fault state. That is, the unit tentatively judges that a fault has occurred in the resolver. When the detection of the square root placed out of the range is continued for a predetermined period of time, the unit judges that the resolver is in a finally-determined fault state. That is, the unit finally judges that a fault has occurred in the resolver. In contrast, when the unit detects that the rotational angle θ repeatedly calculated after the judgment to be in the tentative fault state is changed over the entire range from 0 to 2π radians, the tentative judgment is annulled, and the unit judges that the resolver returns to a normal state. That is, the unit judges that no fault has occurred in the resolver. Accordingly, because the tentative fault state is considered in a state transfer from the normal state to the finally-determined fault state, there is little possibility that a fault of the resolver is erroneously judged.
Assuming that a condition type for returning to the normal state from the tentative fault state is set to be the same as a condition type for judging the resolver to be in the tentative fault state, the unit disclosed in the Publication No. 2006-177750 has the same problem as that in the unit disclosed in the Publication No. H09-72758. That is, assuming that the square root placed within the predetermined range is set as a returning condition, the judgment to be in the normal state and the judgment to be in the tentative fault state are alternatively made in the unit. However, in the unit disclosed in the Publication No. 2006-177750, the rotational angle θ changed over the entire range from 0 to 2π radians in the tentative fault state is set as a returning condition. Therefore, the returning condition type differs from the condition type for judging the resolver to be in the tentative fault state. Accordingly, the repeatedly-alternated judgment between the normal state and the tentative fault state can be prevented.
However, in the unit disclosed in the Publication No. 2006-177750, even if a fault has actually occurred in the resolver, there is a case where the unit erroneously judges that the resolver judged to be in the tentative fault state returns to the normal state. When the unit erroneously judges the resolver actually set in a fault state to be in the normal state, a control such as a motor control using a detection result of the rotational angle detecting device is erroneously operated.